Inorganic substances such as selenium, cadmium sulfide, zinc oxide and amorphous silicon were well known in the photoconductive compositions used in the past for electrophotographic photoreceptors.
These inorganic photoreceptors had the advantage of providing good electrophotographic characteristics, which is to say that they provided extremely good photoconductivity and extremely good charge accepting characteristics and insulation properties in the dark. However, they also had various disadvantages. For example, selenium photoreceptors are very expensive to manufacture and they lack flexibility. They also have the disadvantage of being weak in respect of both thermal and mechanical shock. Cadmium sulfide photoreceptors give rise to problems with pollution since toxic cadmium is used in these materials. Zinc oxide photoreceptors give rise to problems with image stability when it is used repetitively over long periods of time. Moreover, amorphous silicon photoreceptors are very expensive to manufacture and have a further disadvantage in that a special surface treatment is required to prevent degradation of the surface of the photoreceptor.
In recent years, electrophotographic photoreceptors in which various organic substances are used have been suggested as a means of overcoming the problems associated with these inorganic substances, and some of these have been put to practical use. For example, there are electrophotographic photoreceptors consisting of poly(N-vinylcarbazole) and 2,4,7-trinitrofluoren-9-one (U.S. Pat. No. 3,484,237), those consisting of poly(N-vinylcarbazole) sensitized with pyrylium salt based dyes (JP-B-48-25658) and electrophotographic photoreceptors in which the principal component is a co-crystalline complex consisting of dye and resin (JP-A-47-10735). (The terms "JP-A" and "JP-B" as used herein mean an "unexamined published Japanese patent application" and "an examined Japanese patent publication" respectively.)
Furthermore, much active research has been carried out recently in connection with electrophotographic photoreceptors in which organic pigments, such as perylene pigments (e.g. U.S. Pat. No. 3,317,884), phthalocyanine pigments (e.g. U.S. Pat. Nos. 3,397,086 and 4,666,802), azulenium salt based pigments (e.g. JP-A-59-53850 and JP-A-61-212542), squalium salt pigments (e.g. U.S. Pat. Nos. 4,396,610 and 4,644,082), and polycyclic quinone based pigments (e.g. JP-A-59-184348 and JP-A-62-28738) or azo pigments described below are used as a principal component, and many suggestions have been made in this connection.